I am not familiar with push fit waste system but need to fit a new bath to an existing old push fit waste pipe. the final length of pipe is black with the rest grey and goes into a push fit connector. when i twisted and pulled the black pipe, it came out quite easily. is this normal? i'm worried that if i just push it back in again it will leak. i thought push fit pipes aren't supposed to come undone once joined? should i saw it off before the connector and rejoin the pipe using a compression fitting?